Thermodynamics of Electron Transport
In the electron transport chain (ETC), particles move from a negative redox potential (-0.4 V) to a positive potential (+0.8 V). As electrons pass down the chain, their free energy decreases. At certain stages, there is a sharp drop in free energy—specifically at these points (coupling sites) optimal conditions for ATP synthesis are established.
The energy released during the passage of electrons through Complexes I, III, and IV is utilized for the active transport of hydrogen protons ($H^+$) from the mitochondrial matrix into the intermembrane space. This results in the generation of a proton electrochemical potential ($\Delta\mu H^+$), which is driven by the concentration gradient of protons across the inner mitochondrial membrane.
Mechanism of ATP Synthase and Nucleotide Exchange
When the proton gradient reaches a critical level, ATP synthase (Complex V) is activated. A specialized ion channel opens within this enzyme, allowing protons to flow down their gradient from the intermembrane space back into the mitochondrial matrix.
The energy of the electrochemical potential is transformed into chemical bond energy through the phosphorylation reaction: ADP + $P_i \rightarrow$ ATP
For the newly synthesized ATP to be utilized by the cell in energy-requiring processes, it must be exported to the cytoplasm. This is mediated by the ATP-ADP translocase, a carrier protein embedded in the inner mitochondrial membrane. It operates via an antiport mechanism: it exports an ATP molecule from the matrix while simultaneously importing an ADP molecule for subsequent resynthesis.
Oxidative Phosphorylation Efficiency (P/O Ratio)
The efficiency of ATP synthesis is evaluated using the P/O ratio. This is the ratio of the amount of inorganic phosphate consumed to the number of oxygen atoms taken up and reduced to water.
The values of this ratio depend on which coenzyme donates hydrogen to the respiratory chain:
- P/O = 3 (Maximum value): Observed when hydrogen is supplied by NADH. Electrons pass through three coupling sites (Complexes I, III, and IV), each generating a sufficient gradient to synthesize one ATP molecule.
- P/O = 2: Occurs when hydrogen is supplied by FAD-dependent dehydrogenases (e.g., during succinate oxidation). Electrons bypass Complex I, are transferred directly to ubiquinone, and pass through only two coupling sites—Complexes III and IV.
Note: In living cells, actual P/O values are always slightly lower than the theoretical maximums (3 and 2) because a fraction of the electrochemical potential energy is inevitably dissipated as heat.
Reactions at Coupling Sites
Each of the three key complexes performs a specific reaction coupled with proton translocation:
- Complex I (NADH dehydrogenase). Oxidizes NADH and reduces ubiquinone (Q) to $QH_2$.
- Complex III (Q-cytochrome c oxidoreductase). Transfers electrons from ubiquinol ($QH_2$) to cytochrome c.
- Complex IV (Cytochrome c oxidase). Transfers electrons directly from cytochrome c to oxygen, reducing it to form a water molecule.
Complex II (Succinate dehydrogenase) lacks proton-pumping function, thus does not contribute to the gradient and is not a coupling site.
5 Stages of Energy Transformation
Energy transformation in the body, from substrate to high-energy bond, follows a strict sequence of five stages:
- Chemical bond energy of oxidized substances (substrates).
- Electron energy within reduced coenzymes (NADH and $FADH_2$).
- Energy of electrons transferred along the ETC to oxygen (redox potential).
- Energy of the transmembrane electrochemical potential ($\Delta\mu H^+$ gradient).
- Energy of high-energy chemical bonds in the ATP molecule.